Silkscreening
Silkscreening is a printing process that provides the writings and other markings required on the PCB. Silkscreening process used for making PCB Prototypes majorly depends on the quantity and the quality of the final PCBs required. The Silkscreening process used in mass production of PCB Prototypes may greatly vary from the Silkscreening process used for home made or DIY PCB Prototypes. In this post we shall discuss the Silkscreening process which can comfortably be used for mass production of PCB Prototypes as well as production of a few home made PCB Prototypes.
Preparing the screen
A silk screen is used in this process. It is a fabric-like material which is made of polyester or fine silk threads. This fabric is mounted on rectangular or square frame. This fabric is usually prepared to be used in all the PCB processes; etching, solder masking and silkscreening. In this post we shall concentrate on the silkscreening part, although the other processes are done using similar steps.
An emulsion is mixed with developer in a ratio of about 10:1. This is preferably done in a dark room. This mixture is applied evenly on the surface of the silk fabric using a wide squeegee or a similar tool. It is advisable to apply around 2 layers on each side to make a total of 4 layers. This layer must be uniform and there should be no spaces or bubbles. Leave it to dry up or accelerate the process using a drier.
The art work
At this point, the PCB Designs should be ready and printed (black) on a transparent paper.
The designs are placed on the dried silk screen. In this case, the silk screen PCB Design should be placed on the prepared silk fabric the way you would like the writings or the markings to appear on the PCB. It is important to make sure that the PCB Design paper does not move from where you have positioned it. This can be done by fixing the designs firmly on the surface using a TRANSPARENT tape. When this is done, the whole setup (silkscreen and the attached designs) is turned upside down such that the designs face downwards. The top side is then covered with a dark card board. The UV light is then shown from below for about 15 minutes, after when the PCB silk screen art work is fully developed. To expose the developed silk screen template, pressurized water is used to wash away the emulsion parts that were not exposed to the UV (not cured).
Printing
To print the silkscreen on the PCB, the ready PCB (already etched) is placed below the silk screen fabric. The PCB should exactly align with the silk screen art work on top. Again, after positioning the PCB directly below the silk screen and aligning well, the PCB should be fixed firmly in place to ensure that it does not move during the application process. This can be done using stoppers at the sides of the PCB. The silk screen UV curable ink, usually white, is applied evenly on top of the art work (silk screen) using a squeegee. From here, the PCB is dried for about 10 minutes using a drying oven. The writings or markings are well imprinted on the PCB surface.
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